1. Field of the Invention
The present invention relates to substrate transport into processing chambers such as semiconductor chemical vapor deposition reactors, and, more particularly, to a method for increasing substrate throughput and reducing loss of product by reducing thermal shock to the substrate, which can cause breakage and poor quality of the deposited film.
2. Description of the Related Art
In the manufacture of integrated circuits, semiconductor substrates, or wafers, are often processed by chemical vapor deposition. Components of chemical vapor deposition systems include a reaction chamber that is configured to facilitate the controlled flow of a reactant gas and a wafer holder, which is commonly referred to in the art as a xe2x80x9csusceptor,xe2x80x9d for supporting and heating the wafer during processing. To facilitate automated processing, a robotic arm with a wafer handler, or end effector, on the end is employed to place a wafer onto the susceptor for processing and to remove it from the reactor after processing.
In high temperature film deposition or annealing processes, a wafer must be heated to a predetermined temperature after it is introduced into a processing chamber. For example, in epitaxy processes, the temperature is typically around 1090xc2x0 C. to 1190xc2x0 C. This wafer heating can be effected by conduction through direct contact with a heated susceptor, or by radiation through the use of heating lamps.
In current chemical vapor deposition epitaxial reactors, the wafer is typically at room temperature when it is loaded into the process chamber, which is still at a much higher temperature, perhaps as high as 900xc2x0 C. When the substrate is dropped onto the susceptor, both substrate and susceptor experience thermal shock. As its mass is so much less than the susceptor""s, the substrate""s shock is more significant. The thermal effect experienced by the substrate usually varies throughout the extent of the substrate, that is to say that there are large thermal gradients, which may be difficult to mitigate. This can lead to substrate warping and breakage and also to poor uniformity and quality of deposited films.
The susceptor also experiences thermal shock, which is repeated with wafer after wafer, and can ultimately reduce the working lifetime of the susceptor. Usually the susceptor is prepared to receive a wafer by cooling to a temperature much less than the process temperature in order to reduce the thermal shock. Then, once the wafer is in place, the susceptor must be reheated to the process temperature before processing can proceed. This repeated temperature cycling, solely for the purpose of wafer transfer, slows wafer throughput, so most manufacturers compromise by making only small temperature reductions during wafer transfer. This results in less thermal shock but does not completely solve the problem.
There has been some experimentation in the past by customers of the assignee of the present invention in which the heaters for the susceptors were de-energized while a wafer was held above the susceptor for a short interval, before being deposited on the susceptor. While this pre-heating of the wafer reduced shock to the wafer, there was still some warpage of the wafer and the susceptor temperature was dropping during the delay. The temperature when the handler was introduced to the process chamber was probably in the range of about 850xc2x0 C. to about 900xc2x0 C., the temperature of the susceptor and the wafer was probably below 850xc2x0 C. when the wafer was deposited on the susceptor. Also, heat damage to the handler construction limited the temperature that could be maintained.
There is a clear need for a method of wafer exchange in high temperature process chambers that reduces the thermal shock experienced by both the wafer and the susceptor without adversely affecting process throughput, and instead enhancing throughput.
If the wafer can be preheated to a temperature at or near the process temperature before being put on the susceptor, many benefits are accrued. Wafer throughput is increased because no additional time is needed to cool down and subsequently reheat the susceptor during wafer transfer. The temperature remains more uniform because the susceptor remains at or close to the process temperature instead of cycling through cooling and re-heating steps. There is reduced wafer breakage, resulting in less downtime. There is less wafer warping, resulting in better temperature consistency across the wafer and, therefore, better deposited film quality and uniformity. Consumable components, such as susceptors, undergo less thermal shock, thus prolonging their lifetimes and reducing the cost of operating the process equipment.
In general, an embodiment of the current invention comprises a substrate holder configured to support a single substrate for processing in a high temperature chamber. Processing methods that can use the apparatus and method recited herein include chemical vapor deposition, epitaxy, rapid thermal processing, etching, annealing, etc. A substrate handler can pick up an unprocessed substrate, put it on the substrate holder for processing and remove it after processing is complete. Heating is effected by a plurality of heating elements that are positioned to heat the substrate holder, and a control system maintains the substrate holder at a high temperature, such as more than 900xc2x0 C., during wafer transfer. It is preferred that, while holding an unprocessed substrate, the substrate handler pause in a position close to and above the substrate holder for a period of time that allows the wafer to preheat. Preferably, the heating elements are radiant lamps that heat the wafer from above while the hot susceptor is heating the wafer from below, and thus minimize warpage of the wafer.